(1) Field of the Invention
The present disclosure relates to polarization control elements for use in optical disc pickup heads. More specifically, the present disclosure provides multilayer laminates exhibiting wavelength stable retardation for two or more wavelengths.
(2) Description of the Related Art
Optical pickup heads are used both to read and to record information on an optical disc. Generally, this information includes file, audio, and video information. Different laser wavelengths are used, depending on the format. For 650 megabyte (Mb) CD, a 785 nm wavelength is used, for a 4.78 gigabyte (Gb) DVD-ROM, a 650 nm wavelength is used, and for a DVD-R, a 650 nm wavelength read wavelength is used with a 630 nm write wavelength. The proposed next generation DVD media, for example Blu-ray and HD-DVD, use a laser operating at a wavelength of 405 nm. There are several types of optical disc drives in mass production: CD-R (or compact disc recordable), CD-RW (or compact disc rewritable), DVD±R and DVD±RW, which are respectively recordable and rewritable optical discs, as well as next-generation 405 nm drives.
Given that there are so many optical disc formats in existence, the market demands interoperability of several formats in a single disc drive. Accommodation of these several formats, however, often requires numerous sets of optical components of different types configured together. For example, U.S. Pat. No. 6,240,053 to Akiyama shows a conventional pickup head design in which an optical isolator arrangement is used. Light from each laser passes through a polarizing beam splitter and is incident as a linear-polarized incident light on a quarter-wave plate (QWP), which provides circular polarization. Return light from the optical disc, makes a second pass of the QWP, and is thus converted to the orthogonal linear state of polarization relative to the state of polarization of the incident light. This light exits a separate port of the polarizing beam splitter, the light being directed toward a photodetector. As noted by Akiyama, when light of two wavelengths shares the QWP, it is necessary for the QWP component to generate circular polarization at both wavelengths.
Multi-layer retarder stacks that produce a two-or-more wavelength stable responses are well known in the art. In 1948, Destriau and Prouteau combined a quarter-wave and half-wave retarder, with a 60° angle between their optic axes, to produce a circular polarization from linear polarization. In 1955, Pancharatnam combined two half-wave retarders with a quarter-wave retarder to produce an achromatic circular polarizer with a broader spectral range. In the Pancharatnam design, input linear polarized light passed through half-wave retarders at angles of 6.9° and 34.5°, respectively, followed by a quarter-wave retarder at an angle of −79.7°. Mindful that circular polarizers do not behave precisely as quarter-wave plates (i.e., no optic axis), Pancharatnam also generated the design for a three-layer quarter-wave retarder. McIntyre and Harris (1968) disclosed designs for achromatic visible waveplates using a network synthesis technique. Koester (1958) showed that multiple half-wave retarders could be combined to produce broad-band linear polarization rotators. General properties of two-pass retarder networks were also discussed by Ammann in 1966.
Such multilayer retarder stacks have conventionally been manufactured using multiple components with different thermal, optical and structural properties bonded together with an optical adhesive. An example is provided with reference to FIG. 9. As described below, such multilayer retarder stacks are complex to manufacture and are susceptible to deviations in performance caused by temperature variations.
Complexity of known optical pickup head designs leads to increased manufacturing costs and reduced reliability because there is a greater probability of failure as system complexity increases. It is thus desirable to produce optical pickup heads with less expense, and without significantly increasing component count, that can accommodate two or more wavelengths, providing increased compatibility among the various optical disc formats. In configurations using a common path for each laser (or lasers emitting multiple wavelengths), functional requirements of components are thus expanded to cope with certain chromatic effects.